Hitherto, water-absorbent resins have been large in absorption rate, absorption amount, and retention for aqueous liquids and therefore, for the purpose of absorption of body fluids, utilized as components of absorbent structures of the sanitary materials such as diapers by, if necessary, being mixed with fibrous materials such as cellulose fiber, polyester fiber, polyethylene fiber, and/or polypropylene fiber.
In recent years, the ratios of the water-absorbent resins in the absorbent structures tend to increase with the increasing needs of the thinning of the sanitary materials such as diapers (e.g. refer to patent document 1 below). For realization of thinner sanitary materials, it is desired that the fiber materials are replaced with water-absorbent resins for further increase in ratio of the water-absorbent resins in the absorbent structures.
The water-absorbent resin is inherently excellent in the performances of absorbing and retaining the aqueous liquid. However, the fibrous materials are poor in these performances, particularly the performance of retaining the aqueous liquid, and have different performances from those of the conventional water-absorbent resin. As such, as a future water-absorbent resin that responds to the needs, water-absorbent resins having the performances of the fibrous materials in the conventional absorbent structures must be developed. Examples of the performances demanded to such a water-absorbent resin having the performances of the fibrous materials include: a performance of rapidly absorbing an aqueous liquid; a performance of diffusing the aqueous liquid after having absorbed it; and a performance of being capable of temporarily retaining the aqueous liquid after having absorbed it. Therefore, the development of a water-absorbent resins having these performances has been desired.
As an example of such a water-absorbent resin, there has been reported a water-absorbent resin which is not subjected to surface-crosslinking, but coated with cationic polymer forming no covalent bonds on the surface of the water-absorbent resin, so as to realize a low absorption capacity and enhanced liquid-permeability, absorption rate, and wet porosity (see Patent Documents 2 and 3, for example).
[Patent Document 1] pamphlet of WO 95/26209
[Patent Document 2] Specification of European patent application publication No. 1518566
[Patent Document 3] Specification of European patent application publication No. 1518567